So both lights failed the night before the test .... of course they did. It turns out switching the board was actually pretty easy though (took about thirty minutes) so it didn't fail on that.
It did however fail on something quite annoying - missing exhaust heat shield. The tester said he spent some time trying to work out whether it should have one as it would only fail if it had one OEM. He noticed the bolt holes not attaching anything. The manual says: "Only fail a vehicle for missing heat shields if there’s a risk of fire with other fuel system components" - apparently it was because it was between the exhaust and the petrol tank it was a fail.
Of course, a quick post on the 75 FB page was full of "they all rust away and rattle, I ripped mine off as soon as it got the car and it has went through three MOTs since with no advisories". Seems I got the world's most diligent tester ...
Flipping MOT testers doing their job properly😡 how dare they, they should listen to Joe Bloggs off the street who obviously knows best.